Modern integrated circuits are formed by many, often hundreds of, process steps. The formation of each component in the integrated circuit may involve several process steps. For each of the process steps, there is often a plurality of identical production tools for performing the same task in order to improve throughput. However, each wafer is processed by only one of the production tools for one process step.
Currently, the decision as to which production tool a wafer will be sent to is made by a dispatching system. The existing dispatching system primarily makes the dispatching decision based on the production efficiency. For example, a wafer will be typically sent to a production tool that has finished preparation steps, such as pre-heating, and is ready for performing the task. Also, the dispatching system may determine the queuing time of each of the production tools, and send the wafer to the production tool with the shortest queuing time.
Alternatively, a wafer may be dispatched to a production tool having a higher production yield than others. Some production tools, for example, chemical mechanical polishing tools, may generate higher process variations than others. These higher process variations may cause circuit failure for some very small-scale integrated circuits, and yields of production tools may be noticeably different from each other.
The existing dispatching system, however, does not take the within-wafer uniformity of the integrated circuits into account. For high performance integrated circuits, the within-wafer uniformity of the integrated circuits has become an important issue for the stability of integrated circuits. Furthermore, with the down-scaling of integrated circuits, small variations in a physical characteristic may result in a significant variation in electrical performance. New methods for improving the within-wafer uniformity of integrated circuits are thus needed.